Modulated oscillator



f, :UNITED STATE Patented July 15, 1952 s PArsixfr'r `olif-lice- Y f MODULATED osolLLA'roR l Lester M. Field, Palo Alto, Calif., assigner to Bell `Telephone Laboratories,

Incorporated, New

York, N. Y., Ya corporation of New York- Application December 9, 1948.', Serial No: 64,3.-19

-16 Claims.

This invention relates to signaling means and more particularly to ymeans for the generation of frequency modulated electrical waves.

vIt is an object of the invention to provide new and improved methods of and means for producing electrical waves having frequency variations in accordance with modulating signals.

It is another object of the invention to provide apparatus methods of and means for utilizing traveling wave amplifying devices for the generation and modulation of highl frequency electrical waves.

There have been proposed electron beam amplifying devices wherein amplification is achieved by the interaction between the electron stream and apure traveling Wave, that is, a wave which progresses continuously in a'single direction without reflection, along an extended path. Such amplifying devices include awave guidel disposed along the path of the electron stream, the wave guide having an organization such that the travelingv wave has electric eld components in A'the direction of the electron stream and there is substantial equality between the `speed of propagation ofjthe wave in the direction of the stream andr the'speed of the electrons in the electron stream. lThe electron stream then reacts -on the 4 electric field of the travelingwave and the electric field reacts on the electron stream ina manner such thattlie wave increases in'amplitude as it progresses along the path.- Amplifying devices of this 'nature have-been termed traveling wave ampliers and have been'described--for example, in copendi'n'g application Serial No. 640,597, filed January .11', .,1-946 by-J. R'. Pierceand copending viapplicatiori -Selial N'.'704,858, vled Oot0ber2r2,

1946 by Jl R.,Pierce.'

There 'are vdisclosed inlcopending application SerialJ No. 64,309, filed December 9, 1948, by L.

u M. Field; circuit arrangements whereby traveling wave amplifying devices may be utilized for the generatign ofhigh frequency oscillations by utilizinga highjfrequency energy feedback-path vbetween .theijo'utputend and .the input end-ofthe waveguide incorporated in the -device'..t`A-travel. ingwave amplifier maythenbe used' for the genof `an'electron,stream is dependent upon the u configurationand electrical characteristicsof the waveguide.` However, in the presence ofl an electrOnstrea'in the `'phs'i'se velocity of thetraveling Wave istalsoz'dependentuponthe velocity of thev (oi. sez- 7) y electrons of '.-the electron stream. Under such conditions, while the phase velocity of the propagated Wave is different from the velocity of the electrons,v the phase velocity of the wave is increased when the velocity of the electrons is increased and conversely the phase velocity of the y Wave is decreased when the velocity of the electrons is decreased. A

' The present invention contemplates circuit arrangements'whereby the effect upon the phase velocity of the wave produced by changing the electron velocity may be utilized in a feed-back oscillator to provide an improved means forthe generation. of electrical vwaves whose frequencyv is modulated in accordance with arbitrary signals representing speech or other intelligence to be impressed upon the generated waves. Generical- 1y, the invention may be embodied. in a'form such that .the potential gradients along .the path of the electron stream of a traveling waveamplier are varied in accordance with the.. modulating signals to produce an electron velocity variation and a consequent phase variationv of the wave traveling along` the wave guide incorporated in the amplifying device. f There. is thus vproduced an effective change in theielectrical lengthef the loop'circuit vwhereby the frequency of. the generated waves isvaried in accordance .with the modulating signals; More particularly, as applied .to the structure described in the application Serial No. 704,858 by J. R. Pierce `to which reference hasbeenimade, the variable potential gradient may preferably be obtained-by varying the poten- :tial of the helix, which serves as the Wave guide, `-withrespeot to the potentialof the-electron source orcathoden `Alternatively the modulating potentials vmay be utilized tol vary thev potential of either the focussing electrode-of the electron gun or the collector anode with respect to the potential of the cathode. kOther possible `embodiments Aof the invention` are pointed out in connection with the'detailed description of thefinvention.`

The features of this invention are pointed out with particul'arity in the appended claims. The

invention itself may 'be kbetter understood' by reference to the following .detailedI description taken in connection with the accompanying drawings in which: u. f

' Fig. l'fillustrates an embodiment of" theinvention YutilizingY then'preferred lform ofI amplifying device; a. Y I

Figs.l i2" and 3 illustrate 'alternative Vcirouit'arrangements useful in connection withv the embodiment of Fig. 1-;1Y andrav .Y

Fig. 4 visa graph useful in explaining the lprinciples ofope'rationof the cirouitsof the invenftion.. f

'Referring'no'w to Fig. 1,'there is 'shown anillusy l 4'trative embodiment of the circuit ofthe invention 3 adapted for use at hyper and ultra-high frequencies. The arrangement shown comprises an electron beam 'tube' I0 including an'evacuated'envelope'rhaving an elongated portion II and an enlarged electron-gun-containing portion I2.

The envelope is constituted of a 10W loss insulating material such as quartz or glass.

The discharge device I0 is provided'with a source of electrons together With means'ior'forming the electrons from the Soiirdeintoav 'concen-itrated stream directed axially-'throughan el'ongated helix I3 disposed Within the elongatedpor- The 'illustrative combition II of the envelope. f nation includes a known type of electrongun It comprising a cathode I5, a heater I6, and a They focussing celerate and focus electrons fromthecathode I5 into a concentrated stream. The electron stream 'so ormed'moves'along an axial path through the helix I3to a collectorV anode 2 0,' thehelixand the "col-lector anode being-maintained at suitable positive *potentials by the' source I9.

'Ihe ele'ctron stream is `further 'concentrated and guided along the axial path by' a magnetic Iield formed byV a f coil 2 I *which isienergized'by a lsource'22 through a rheostat 23. AThe strong magnetic eldserves also to prevent "therdeviationof'the' electron stream from the desired pathby outside-magnetic inuences. The elongated'helix I3'servesf'as'a wave frequency Wavesmay 'be propagatedto interact with the 'electron streamiflovving' therethrough. In order to perform this function-the ldimensions ofthe helix must'befsuch that fthe Waves'vvill' be propagated 'in'fa modev havingffel'ectr'c -eldfcornponents in the'direction of the electronstream and at aspeed along =the` axis of the-'helix 'of an order to Which'the electron stream'mayfbe ac-`v celerated by the vuseof moderate voltages.' f ,For

acceleratingV voltages in. the order'of'lLGO to 2,000 volts, the'helix'm'ay be Wound'w'iths'everal turns per Wavelength 'along the axis. Theinteraction between theelectronfstream fand'the 'travr eling VWave isv cumulative land to vinsure an'adequate interaction',"the axial length of fthehelix may be of the order of thirty to .fortyv Wavelengths.

There are provided Wave paths" external'to `the amplifying device-I0 for coupling'waves'or impulses tobe vamplified jtofthe helix '"-I 3" andfor transferring todo'ad4 or* other` utilization icircuits the ampliedlwave'appearing at 'theoutput end of the'helix' At' the input end'of'fthehelixV I3, an input circuit orwave path is conventionally represented `as a 'Wave' guide"24" of' rectangular cross-section. The amplifying 'device I0 is vin- ,serted transversely through the 'inputwave guide 24,' coupling between 'the-guide" and'the'helix.' I3

trans-V l mission path oriWa've i guide 'along' which high mission line thus formed acts as a 10W' impedance path across the opening in the wall of the Wave guide through Which thee'nvelope I I" is inserted and as a vlow 'impedancefsupport point for the coupling strip 25. Similarly, at the output end 'of the helix the amplifying device I0 is inserted transversely through an output Wave guide 28.

"An output coupling strip 29 is conduotively conn'ected tothe helix I3 and is supported by a cylinflo.-

-fdricalmetallic section 30 Which cooperates with ari external metallic section 3| attached to the output Wave guide 28. InV general,the couplings between' the helix I3 and the input andoutput Wave' guides 24 and 28 may be proportioned in accordance With the considerations set forth in *application Serial' No. 705,181, filed October 23,

1946 r-by W. Wu Mumford-for optimum response over a Wide range of operatingffrequencies. fIK-hus thepitch of the turns on the helix .I3 may be gradually increased over several lturns at each endftovrassist in'matchingthe-impedancesat the couplings over a Wide band of frequencies# Necessary tuning of the--external-wave paths may be achieved by Ameans -suchas atuning screW- 32 illustratively shown inconnection-vvith. the input -vWave guide 24.

The helix I 3 is supported by a series Y(,egfqthree or four) 'of Ynon-conductive ceramic rods 33,5 one 'only'being shown; Which are disposed between the --helixl and the envelope II- and which-V-alsofcoopy'erate with ceramic spacers-34`and35 te-maintain aiongitudinal--spaced relation-between-the'focus- -`sing electrode f I'I,` the input couplingfstrip Vsupport section 26, theoutput coupling` strip-support sectiont 30,- andthe-collector anOde-ZBQ- JIhe 'ceramic rods-33 are coatedwith a-wavefdissipative materia-lD in order'todissipate Waves travelingalong the-helixVr I3 in a `direction-oppositeto that of the v`electron stream, the'dissipativematerial and the rods'being organized-infaccordance with the considerations -set forthdn application Serial` No'. -7,04;198,fled October 22,` 1946, now-United yStates j'Patent" 2,575,383.'l fdated yNovemberl 20,- 1951 by L.-M.-Field.

, y Afhigh frequencyfenergy feedback pathisg-providedv between lthe output wav@ guideV` 28` and the input-waveguide 24ftofform a feedback-'oscillay-tor'circuit infaccordanc'e with the considerations set forth'in the previously referencedapplication Field'. *The high frequency energyl feedback-path is conventionally represented las "sectionsf361 and 31 of coaxial transmission line coupled-through 'aknown 'type of phase' shifter"38,`andextending between the 'output' waveguides-Maand the input transmissionlineV 364 is extended into 'the interior of `the 'output 4wave 'guide'y 28 ".tozform' 4affcoupling (5o-'transmission line 31 is vexten-ded:intothe-interior being achieved by means of an input Vcoupling strip: 25V 'conduotively connectedto -thezinput end of the helix. zTh'erexis :provided a"cylindrical4 metallic section 2S Whi'oh'supports the-"inputcou A plingl strip7 25fand cooperates 'with ua; cylindrical "metallic-section'121r attached to lthe inputwave guide 24 to eieotively form an open circuited Y`coaxial transmissionline which.z has anele'ctrical v- -length-infthe orderion'onefomfth of.- the Wavewlengthxof the fwaves :to be' ainpliiied. The rtrans- 4frequency 'energy'feedba'ck path tossetfupgc'orresponding elds in the input circuit formed jby famplie'dby interactionrvvith' the electronlgstream Y vandtof-produce'-continuous electrical oscillations.v

'i The phase'shifter 38 is rinclu'dedinthe high requencyenergy feedbackibathto provide'farepre- -f'sentationof Ea necessaryfunctional elementrof-t-he feedback oscillatorloillustrated, l althoughvf'the Vphasegshifter needanotjnecessarilybepresentias av 1 separate entity in a practical embodiment of the oscillator. The necessary phase shift is ydetermined by the frequency of operation desired and may be achieved in accordance with the considerations set forth inthe above referenced application by L. M. Field. I

There is provided in accordance with the invention a source of modulating waves or signals conventionally represented as. an alternatingk currenttgenerator 43. The source of signals may supply variable potentials or currents of a single frequency, or more commonly, a complex signal having a spectrum including many components such as is: customarily utilized in the transmission of speech or visible images. Signals derived from the source 43 are applied through a coupling means, conventionally represented as av transformer 44, connected between the conductor 45 and the-potential source I9, thus effecting a series connection between the cathode I and the potential source. The focussing electrode I1, the helixv I3, and the collector anode 20 are connect'edto` positive poles of the potential source I9- byA means of` conductors 46, 41, and 48 respectively, sothat the potentials of each; while fixed with respect to the others, are simultaneously VariedY with respect to the cathode I5 in accordance with the amplitude variations of the modulating'signals. The resulting vvariation of the poyThe l'marmer of! operation of the circuits of the ,Y

invention may be most readily understood whilel having in mind-the pertinent operational aspects of the -traveling wave amplifying devices used in those circuits. Briefly, the helix I3 of the illustrative device Il! of Fig. 1 acts as a wave guide for waves of proper frequencies, the waves being propagated along the turns of the helix in a mode such that the electrical field associated with the traveling wave has components in the direction of the electron stream. By reason of the fact that the-wave travels along the circumference of the helix, the axial velocityof the wave is such that there is substantial equality between the speed 'of the electrons in the electron stream and the speed of propagation-'of the wave. Under such conditions the interaction between the electron stream and the wave increases the amplitude of the wave as it progresses along the helix.

A wave,in traveling over the helix from the input coupling strip l25 to the output coupling strip 29, will, in the absence of the electron stream, undergo a change in phase determined solely by the geometry and electrical characteristics of the helical wave transmission path. However, in view of the electrodynamic aspects of the wave as it propagates along the helix in the Apresence of the electron stream, the total phase shift of the wave in traversing ythe helix in the presence of the electron stream is also dependent upon the velocity of the electrons. Thus for every electron stream velocity as determined by the-potentialsfapplied to the focusing electrode I1, the helix I3, and the collector anode 20,

there correspondsa value of phase shift experienced by a wave of a particular frequencyinv `traversing' the helix: Further, the total phase shift :of' the wave in'traversing the helix will be alinear function of frequency, at least over re L striced ranges of frequencies.. c

The principles of operation of the feedback oscillatorof Fig. 1 have been described in detail in theL. M. Field .application to whichl reference has previously been made. For the present pury pose it may suffice to say that waves of proper frequencies, which may originate as random 'impulses, may successively traverse the loop `formed by the helix I3, thevoutput Wave guide 28, the high frequency energy feedback path comprising the transmission lines 36 and 31 andthe phase shifter 38, and the input wave guide 24. The amplitude of such waves is increased for each successive passage around the` loop, the waves formingcontinuous. oscillations which reach a limiting amplitude dependent-,upon the voltage and current characteristics :of the amplifying device.

The waves which'iare'builtup into continuous.

oscillations are waves ofa frequency such, that they undergo a total phase shift which Y,is an integral multiple of 21rv radiansin traversing the loop from a starting pointsuch: asv thefoutput coupling strip 29, through the output waveguide 1 28, the high frequency energy feedback path, the Y input wave guide 2.4, the helix i3, andback to the starting point. This phase shift maybe termed the' loop phase shift as distinct from lengthof the loop the frequency will change cor-VV respondingly. This will bemade moreclear froma consideration` of Fig. 4 where the functional `relationships between :the phase shift around the loop, plotted as an ordinate, and the frequency, plotted as -an abscissa are set forth in graphic fashion." The 'solid curve 52 indicates-the loop. phase shift as a straight Vline or linear function of frequency, this representation -being correct for at least limited ranges of frequency variation, the slope of the curve beingdetermined by the total phase shift around theloop. The points along theordinate corresponding to phase shifts of integral multiplesof 21rradians then determine the possible frequencies or Vmodes at which oscil-4 lation may take place, the phase. shift for` 2(N-}2)1r radians being arbitrarily selected as corresponding to a frequency f, which may be termed the carrier frequency.

In accordance with the invention, variations of carrier frequency corresponding to thevvariations of the modulating signals are achieved by yvarying the velocity of the electronstream of the amplifying device I3. It will be apparent that, since the totaly loop phase shift must remain constant (i. e. fora givenfmode yof oscillation) and since` the phase shift of waves traversing the helix is a function of the frequency of the wave, as the velocity ofthe electron stream isvaried the frequency of oscillation must also vary in order that the loopphase lshift yremain constant.` Thus for oscillations corresponding to a loop phase vshift of 2(N+'2)1r radians the oscillation frequency is varied between the limiting frequencies f1 and f2, the limiting variations indicated by the lines 53 and l54 being determined by the amplitude-of the yand frequency of 'y feedback path whereby the frequency of the said l into a stream traveling lengthwise of and in the field region of said helix, a collector electrode, a potential source having a negative pole connected to the said source of electrons and positive poles connected to the said helix, the said focussing means and the said collector electrode for establishing a potential gradient along the path of the said electron stream, means for coupling high frequency energy from the said output end to the said input end of the said helix so as to form an oscillation generator, a source of modulating signals, and means for varying the velocity of the electrons of the said electron stream in accordance with the variations of said modulating signals.

9. The invention in accordance with claim 8,

Vsaid velocity varying means comprising means 11. The invention in accordance with claim 8,

said velocity varying means comprising means for varying the potential of the said helix in ac-` cordance with the variations of the said modulating signals.

12. A frequency modulated oscillation apparatus comprising a traveling wave amplifier, a high frequency energy feedback path connected between an input end and an output end of said traveling wave amplifier whereby high frequency electrical oscillations may be generated, a source of modulating signals, and means responsive to the signals from said source for varying the frequency of said oscillations in accordance with the variations in the amplitude of said modulating signals. j

13. A frequency modulated oscillation apparatus of the traveling wave amplifier type in which a stream of electrons is directed along a distributed wave transmission circuit in coupling relation thereto at a velocity substantially equal to the velocity of Wave propagation along the circuit, and in which amplified wave energy is fed back to an'input portion of said circuit to achieve tions, a source Vof modulating signals and means responsive to the signals from said source for varying the velocity of said electron stream in accordance with the variations in the amplitude of the modulating signals, whereby the frequency of said oscillations is varied in accordance therewith.

15. A frequency modulated oscillation appara-` tus comprising a distributed wave transmission circuit, a source of electrons, means comprising a direct current potential source for producing a unidirectional electric field in the region of the cathode and the Wave transmission circuit` to form electrons from said source into a vstream traveling along said circuit at a velocity substantially 'equal to the velocity of wave propagation along the circuit, whereby an electromagnetic wave traveling along the circuit in the direction of the electron stream is amplified, feedback means for transferring amplified electromagnetic wave energy from a portion of the circuit remote from the electron source to a portion of the circuit nearer to the electron source whereby the generation of high frequency electrical oscillations is achieved, a source of variable potential modulating signals, and means responsive to the modulating signals for varying the said unidirectional electric field and thereby the velocity of the electron stream in accordance with the variations in amplitude of the modulating signals, whereby the frequency of said oscillations is similarly Varied.

16. A frequency modulated oscillation apparatus of the traveling wave amplifier type in which generation of highl frequency electrical oscillavelocity of wave propagation along the circuit,

and means for feeding back amplified wave energy to an input portion of said circuit to achieve generation of high frequency electrical oscillaa stream-of electrons is directed along a distributed wave transmission circuit in coupling relation 'thereto at a predetermined velocity, and in which amplified wave energy is fed back from one portion of said circuit to another to achieve generation of high frequency oscillations, including a source of modulating signals and means to Vary the Velocity of wave transmission along said circuit under the control of the modulating sig- .nals whereby tosignal-modulater the frequency of oscillation, said wave velocity variation means comprising said source and means coupled thereto to vary the velocity of said electron stream over a range of velocities within which said coupling relation obtains, in accordance with the variations in amplitude -of the modulating signals.v

, LESTER M. FIELD.

REFERENCES CITEDv The following references are of record in the file of this patent:r

UNITED STATES PATENTS OTHER REFERNCES Article, Wideband Microwave Amplifier.

Tube, pages -92 of Electronics for November 1946.

Article, The Beam Traveling-Wave Tube, by

J. R. Pierce, from BellLab. Record for December Article, The rJraveling-ylave Tube as an Amplifier at Microwaves, by Rudolph Kompfner, from Proceedings of I. R. E., vol. 35, No. 2, February 1947. 

